UNIT 2 – ENERGY and MOMENTUM
CHAPTER 5 – Momentum
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5.1 Momentum and Impulse
o Momentum is the product of the mass of a moving object and its velocity
 p = mv
 where p is momentum of an object in kg m/s
o
the force applied and time to apply the force affect the momentum
 using Newton’s 2nd law
o
the net force on an object equals the rate of change of the objects momentum
 Ex. 1. A hockey puck with a mass of 0.83 kg is passed across the ice with a force
of 16.3 N. Initially the puck has a speed of 18 m/s and a coefficient of kinetic
friction of 0.4
 a. Calculate kinetic friction
 b. Calculate net force
 c. Determine the initial momentum of the puck
 d. Determine the final velocity of the puck as it travels for 1.37 s
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5.2 Conservation of Momentum
o Law of conservation of linear momentum
 If net force acting on a system of interacting objects is zero, then the linear
momentum of the system before the interaction equals the linear momentum
of the system after the interaction

Ex. 1. You hit the white ball with a speed of 4.6 m/s against a somewhat
stationary 8 ball that then travels with a speed of 3.2 m/s. After the collision the
white ball travels with a speed of 1.4 m/s in the opposite direction and all the
balls have an approximate mass of 2.2 kg. Determine the initial speed of the 8
ball.
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5.3 Elastic and Inelastic Collisions
o Elastic collision – the total K.E. after the collision equals the total K.E. before the collision
 Almost impossible to occur because after collision some energy is always lost to
surroundings
o
Inelastic collisions – the total K.E. after is different from the total K.E. before the
collision
o
Complete inelastic collision – there is a max decrease in K.E. after collision since objects
stick together and move at the same velocity

Usually the final K.E. is less then initial K.E. unless collision is explosive
 The decrease in K.E. is due to transfer of energy
o Ex. To thermal (heat) energy
 As K.E. decreases, elastic P.E. increase
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For inelastic collisions
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i.e. conservation of momentum
For complete inelastic collisions
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For elastic collisions
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Ex. 1. A truck of mass 1.3 x 104 kg travelling at 90 km/h [N], collides with a car of
mass 1100 kg travelling at 30 km/h [N]. If the collision is completely inelastic,
what are the velocities of the vehicles after the collision?